In the recovery of oil from subterranean, oil-bearing formations, it is usually possible to recover only a limited proportion of the original oil present in the reservoir by the so-called primary recovery methods which utilize the natural formation pressure to produce the oil through suitable production wells. For this reason, a variety of supplementary recovery techniques have been employed, directed either to maintaining formation pressure or to the displacement of the oil from the porous rock matrix. Techniques of this kind have included formation pressurization, thermal recovery methods such as steam flooding and in situ combustion, water flooding and miscible flooding techniques.
One of the flooding techniques which has been investigated is carbon dioxide flooding and this is considered to be a method of substantial promise. In the carbon dioxide flooding technique, a slug of carbon dioxide is injected into the formation to mobilize the oil and permit it to be displaced towards a production well at an offset from the injection well. Carbon dioxide is not miscible in crude oil under normal conditions because the pressure at which it becomes miscible with most reservoir oils is generally greater than about 7,600 kPa (about 1,100 psia) . However, under supercritical conditions, usually of high pressure, carbon dioxide acts as a solvent and in certain reservoir situations has a great advantage over more common fluids as a displacement agent. Even under conditions where the carbon dioxide is not wholly effective as a solvent for the oil, recovery may be improved by taking advantage of the solubility of carbon dioxide in the oil, causing a viscosity reduction and a swelling of the oil, which leads to increased recovery. These effects have been utilized at pressures much lower than the miscibility pressures for carbon dioxide for oil. Processes using carbon dioxide as a recovery agent are described, for example, in U.S. Pat. Nos. 3,811,501, 3,811,502, and 4,410,043.
One problem which arises from the use of carbon dioxide as a flooding agent is that it is much less viscous than oil or water and the result of this is that the injected fluid does not displace the oil uniformly. Instead, the carbon dioxide moves faster in some regions and directions than others and "viscous fingers" are formed through which most of the injected fluids flow. Some of these fingers may arrive prematurely at the production well, lowering the effectiveness of both the injected carbon dioxide and of the production pumping capacity.
Several general methods have been proposed for controlling the mobility of carbon dioxide when used as a flooding agent so as to maintain the desired profile in the front between the slug of carbon dioxide and the oil in place in the reservoir. In general, the methods used or proposed for the control of frontal instability entail the increase of the flowing pressure gradient behind the front, that is, a decrease in the displacing fluid's mobility. An initial proposal to decrease the effective mobility of the displacing fluid, so as to increased the pressure gradient in the region it occupied was to add water to the injection fluids in the process known as WAG, or water alternated with gas. Although this procedure has been adopted in a number of applications, there are problems with its effectiveness. First, the injected water may prevent the oil forming good contact with the displacement fluid and second, the gravity segregation of the water and gas results in the more dense water flowing in the lower zone and the less dense carbon dioxide preferentially flowing in the upper zone of the reservoir so that the potential advantages of the process were vitiated. A second proposal has been to use a foam-like dispersion of carbon dioxide in a surfactant solution because composite fluids of this kind would have a decreased mobility through porous rock, apparently as a consequence of the formation and migration of the aqueous films in which most of the water present in the foam is transported. These films would separate the carbon dioxide into cells, increasing its resistance to flow in the porous medium. It has been known for some time that a surfactant stabilized foam of gas and water has a very low mobility to flow through porous media such as the porous rock matrices of subterranean, oil-bearing formations. Because the foam is a composite fluid with a structure comparable in size to the average pore size of the rock matrix, the mobility in the rock cannot be assumed to be capable of calculation as the ratio of rock permeability to a fluid viscosity. Despite this difficulty, however, the mobility reduction has been measured under reservoir conditions in the laboratory and measurements utilized for prediction of behavior in the field. However, the methods previously used for determining the foaming ability of a foamant solution and the stability of the foam generated have been inconvenient and have required correlations and assumptions in relating conditions measured in the laboratory with those which are likely to exist in the reservoir.
A number of tests have been proposed in the past, including the standardized method known as the "Ross Miles Technique" described in "Foaming Agents: Cure for Waterlogged Gas Wells" Dunning, H. N. et al., Pet. Eng. November 1959, pp b28-b33. This test was, however, envisaged merely as a screening test and the requirements of mobility control in carbon dioxide floods have imposed other constraints on the foam generation than simply that they must be producible at ambient conditions. These constraints include chemical compatability with carbon dioxide and, to some extent, with crude oil. Tests of foam generation and stability under reservoir conditions of temperature and pressure are therefore needed. While pressure is apparently innocuous to the mobility reduction properties of foams in porous media, the high pressures which are prevalent in reservoirs may impose a different environment which may alter foaming characteristics; for this reason, pressure is a significant consideration and requires to be taken into consideration in any test for the utility of a foamant. Any test for the suitability of a foamant under practical conditions therefore requires its effectiveness to be directly examined. Thus, measurements should be made of mobility reduction in different porous media.